Drying apparatus and process



'April 28, 1970 1W, NEBL fT ET AL 3,508,339

' DRYING APPARATUS AND PROCESS I Filed Nov. 30, 1965 2 Sheets-Sheet 1 51/1. /v 55 zrmkw A7 O/I'W') J. w. NEBLETT ET'AL.

DRYING APPARATUS AND PROCESS April 28, 1970 2 Sheets-Sheet 2 Filed Nov.50, 1965 vwavrazm: J4M5 M N53! [/72 20/2440 5 DFEAC/vffi, 5/14 r 4; assafir j ram [r United States Patent 3,508,339 DRYING APPARATUS AND PROCESSJames W. Neblett, Donald E. Debacher, and Billy A.

Nussel, Mount Vernon, Ind., assignors to General Electric Company, acorporation of New York Filed Nov. 30, 1965, Ser. No. 510,554 Int. Cl.F26b 3/10 US. Cl. 34-10 3 Claims ABSTRACT OF THE DISCLOSURE A dryingapparatus capable of providing a feed material in the form of dense,discreet particles wherein the apparatus has at least one drying stagewhich has in combination a jet mixer and an elongated drying pipe incommunication therewith. The jet mixer has a hot gas entry duct, a jetnozzle having a Reynolds number of at least 10,000, a mixing chamber atthe discharge end of the jet nozzle which has means for the introductionof a feed material to the discharge end of the jet nozzle at an angleconverging with the discharge end thereof. At the discharge end of themixing chamber is a diffusion zone, which diifusion zone diverging at anangle that allows limited expansion of a gas stream passing therethroughwhile maintaining contact between the gas stream and the walls of thediffusion zone. The gas entry duct, jet nozzle, mixing chamber anddiffusion zone are in axial alignment. Further the invention is directedto a process for drying a material in the form of dense, discreetparticles, which process comprises continuously passing a stream of hotgas through a jet nozzle at acoustical velocity, angularly projecting afeed material into the stream of hot gas as it discharges from the jetnozzle to form a suspension thereof in the hot gas, passing thesuspension into a confined diffusion zone in order to allow limitedexpansion thereof while maintaining contact with the walls of thediffusion zone, advancing the suspension through an elongated pipe andthen separating volatiles from the formed particulate material.

This invention relates to a rapid drying apparatus and method wherein adry material is obtained in the form of discreet, dense particles ofpredetermined particle size. More particularly, this invention relatesto a rapid drying apparatus consisting of one or more mixing and dryingstages and to a method wherein a wet particulate solid or solution isprojected into a high velocity stream of a hot gas or vapor in aconfined chamber and thereafter passed through a drying pipe to formdiscreet, dense particles of predetermined particle size.

An important operation in many plants related to the chemical andplastics industry is the separation of a dissolved, solid material fromits solvent while at the same time controlling the resultant particlesize and particle form so that the precipitated powder will be suitablefor further processing. Many processes and apparatus have been developedfor such operations but in general, the dried powder resulting from suchoperations is usually in the form of small, porous agglomerates that aredifficult to handle. One such prior art operation identified as spraydrying involves atomizing a solution of the material to be dried intovery small droplets dispersed in an atmosphere of hot gasses in a largeopen chamber. The solvent is rapidly removed due to the extremely small"ice size of the droplets and the resulting large total drying surface.One difficulty encountered with the process is that many materials, suchas resin solutions, become case hardened when spray dried. On drying,the resins form a tough outer skin which is subsequently blown apart asthe interior liquid vaporizes. This forms hollow, fragile sphericalparticles which have low bulk density. Also, the particles are oftenvery small, resulting in a material that is too dusty for subsequentmaterial handling equipment.

Another frequently utilized drying operation is that of prilling. Thisoperation is a combination of spray drying and crystallization. Largedroplets of a hot concentrated solution are sprayed into a tower andallowed to cool on falling through hot air blown through a tower. Thesolids crystallize into agglomerates or prills which are generallyspherical in shape. This method is limited to materials which can behighly concentrated in hot solution and which readily crystallize oncooling. Many materials, such as plastic resins, do not crystallizereadily and their concentrated solutions are too viscous to be sprayedat temperatures below degradation levels. Materials of this naturerequire relatively concentrated solutions for efficient handling. Dilutesolutions would require infinitely large prilling towers.

A third procedure frequently employed to recover a solid from itssolution is the precipitation technique. In this procedure, anon-solvent for the solid to be collected is added to a solution. Thenon-solvent is substantially more miscible with the solvent than withthe solute and thus causes the solute to separate from the solution as asolid phase. The resulting slurry can then be filtered or centrifuged torecover the solids as a wet cake. This technique has two distinctdisadvantages. First, a small fraction of the solids will remaindissolved in the resultant liquid phase and generally, will be lost fromthe product yield. Second, the original solvent and the non-solvent mustbe separated if they are to be resused in the process. This generallyinvolves several additional operations such as evaporation, fractionaldistillation, and extraction. In addition to this, the solute isgenerally collected in the form of a wet filter cake which must becrushed, dried and sieved to obtain a powder which may be utilized insubsequent operations.

Another approach, known as jet spray drying, involves the introductionof a solution through a water-cooled tube axially aligned with a gas orsteam discharge nozzle. The gas or steam leaves the nozzle at highvelocities causing the solution to be atomized and entrained by the hotgas or steam. The nozzle discharges into a large open area and thesolvent is evaporated. The solids phase is separated from the gas phasein an apparatus such as cyclone separator. A major disadvantage of thejet spray dryer is that the particles of solid are very small, usuallyless than 10 microns. This offsets the efficiency of the cycloneseparator and subsequent handling equipment and necessitates anadditional agglomeration step to re-group the tiny particles intolarger, more handleable ones. Usually, this involves the reintroductionof a small amount of solvent as a spray to cause the smaller particlesto agglomerate.

It has now been discovered that the disadvantages of the various dryingprocedures noted above can be substantially eliminated by the practiceof the present invention which comprises passing a stream of a materialto be dried through one or more drying stages wherein each drying stagehas in axial alignment a jet mixer and an elongated drying pipe. Byemploying the apparatus and procedures of this invention, it is possibleto form dry, discreet, dense particles of a material having apredetermined particle size from its solution. It is also possible torapidly form dry, discreet, dense particles from a wet particulatesolid. In addition, materials may be dried that have low decompositiontemperatures because residence time in the apparatus is usually lessthan 60 seconds. A further advantage of this system is that theelongated drying pipe may be used to simultaneously dry and conveymaterial from one part of a plant to another.

Accordingly, one object of this invention is to provide a rapid processfor forming discreet, dense particles of a material from its solution.

Another object of this invention is to provide a rapid process fordrying a wet particulate solid.

A third object of this invention is to provide a drying process whereinthe particle size of the resultant dry solid may be controlled andpredetermined.

A further object of this invention is to provide a drying apparatuscapable of forming discreet, dense solid particles of predeterminedparticle size and consisting of a jet mixer and a drying pipe.

An additional object of this invention is to provide a drying apparatusconsisting of a plurality of stages.

Other objects and advantages of this invention will be in part apparentand in part pointed out in the description which follows.

Briefly stated, the objects and advantages of this invention areachieved by passing a stream of a material to be dried through one ormore drying stages wherein each drying stage consists of a jet mixer inaxial alignment with an elongated drying pipe. The material to be driedmay be in the form of a solution or wet particles. For brevity, thematerial to be dried will hereinafter be referred to as the feedmaterial which is defined to include both solutions and wet particles.

The jet mixer for each drying stage consists of a jet nozzle in axialalignment with a confining mixing chamber and a diffusion zone. A streamof high velocity gas is passed through the jet nozzle and into themixing chamber. The feed material is forced into the mixing chamber atan angle converging with the hot gas stream. When the feed material isin the form of a solution, it is sheared or cut into small slugs ordroplets which are suspended and intimately dispersed in the hot gasstream. This suspension is passed through the diffusion zone and into anelongated pipe. The suspended droplets are twisted, stretched and tornapart by the turbulent action of the high velocity gas stream. Thistreatment exposes considerable surface area of the droplets to the hotgas and promotes rapid vaporization of the solvent. It also tends tooffset case hardening of the particle since the continually changingshape of the droplets allows internal vapors to escape readily. At somepoint downstream, the droplets of solution will have lost enough solventto become tacky particles which reagglomerate into large denseparticles. These dry further downstream and can be separated in acollecting apparatus such as a cyclone separator.

During the course of the drying process, the hot gas stream becomessaturated with volatiles and drying efficiency is substantially reduced.To avoid this, it is desirable to collect the particles in amoist-non-tacky condition and pass them through a second jet mixer andinto a second drying pipe. This procedure can be repeated as many timesas is necessary to effect complete drying.

'When the feed material is in the form of a solution, particles ofpredetermined size can be obtained by adjustment of the ratio of therate of flow of the hot gas to the rate of flow of the feed materialand/or by control of the percent solids in the feed material. It hasbeen 4 found that decreasing the .above noted ratio or increasing theconcentration of dissolved solids favors the formation of largeparticles.

This invention can be better understood by reference to the drawingwherein:

FIGURE 1 is a sectional elevation of one embodiment of a jet mixersuitable for forming a suspension of a feed material in solution formand a hot gas stream consisting of a jet nozzle, a mixing chamber and adiffusion zone;

FIGURE 2 is an elevation of the apparatus of this invention includingthe jet mixer of FIGURE 1, a jacketed drying pipe and a cycloneseparator; 7

FIGURE 3 is a sectional elevation of another embodiment of a jet mixerdesigned for mixing wet particulate material with a stream of highvelocity, hot gas, and

FIGURE 4 is an elevational view of a two-zone drying apparatus inaccordance with this invention and includes a first and second stage jetmixer, a first and second stage drying pipe and a volatile recoverysystem. The jet mixing apparatus depicted in FIGURE 1 includes a gasentry duct 1, a convergent-divergent jet nozzle 2, a mixing chamber 3,and a diffusion zone 4 consisting of a diffusion throat 5, and diffuser6. The hot gas stream enters the mixing apparatus through the gas ductand its velocity is substantially increased as it passes through the jetnozzle and into the mixing chamber. The feed material 7 is forced intothe mixing chamber 3 through entry duct 8. It is necessary that thesolution enter the mixing chamber along an axis converging with that ofthe hot gas stream. In other words, the solution may not enter themixing chamber in axial alignment with the hot gas stream. The angleformed between the axis of the gas stream and that of the solutionpreferably varies from approximately 30 to As the feed enters the mixingchamber, it is sheared into small slugs or droplets by the high velocitygas stream and a suspension of the liquid in the gas stream is formed. Asuspension of this nature does not form when the solution enters themixing chamber in axial alignment with the hotgas stream. The dropletsof entrained solution are twisted, stretched and torn apart by theturbulent action of the hot, high velocity gas stream and a considerablesurface of the droplet is exposed to the hot gas. This promotes rapidvaporization of the solvent and prevents case hardening of the particlesince the continually changing shape of the droplets allows internalvapors to escape. readily. In order to insure that sufficient surfacearea of the droplets is exposed to drying, the hot gas must bemaintained at a high velocity and it is necessary that the Reynoldsnumber for the jet nozzle exceed 10,000. The diameter of the jet nozzleat its narrowest point may vary between broad limits and is dependentupon thecapacity and size of the entire drying system. It has been foundthat the diameter may vary between 0.10", and 1.50" for a system havinga 3" inside diameter gas entry duct 1.

In order to prevent sedimentation formation in the inlet 8, it isnecessary to position a heat insulating material 9 between the jetnozzle- 2 and the feet inlet 8. Sedimentation formation can forceshutdown of the system due to blocked passages and also causes oversizedlumps in the power product. Any heat insulation known to those skilledin the art may be employed provided it is non-reactive with both thesolution to be dried and the hot gas stream. An excellent heatinsulation material has been found to be tetrafluoroethylene.

The hot gas stream with its entrained solution expands in the diffusionzone 4. It is necessary that the diffusion zone confine the gas streamat all times while allowing for limited expansion. Therefore, thediffusion zone diverges at an angle which provides for limited expansionwhile maintaining the hot gas stream in contact with the walls of thechamber. For most systems, it is desirable that the angle formed betweenthe axis along which the gas stream passes and the walls of thediffusion zone vary between 3 and 12. It is through the maintenance ofhigh turbulence and velocity caused by the confined area of thediffusion zone that the discreet, dense particles of predeterminedparticle size are formed.

The suspension of hot gas and entrained solution next passes from thedischarge end of the jet mixer into a drying pipe 10. The boundarybetween the pipe and the diffusion zone should be free of geometricirregularities and the diameter of the pipe should preferably be of thesame diameter as the discharge end of the diffusion zone. This diametermay vary between board limits depending upon the capacity of the systememployed. If geometric irregularities occur between the diffusion zoneand the pipe, a viscous material may be trapped and solute willprecipitate in the form of lumps that eventually collect with the driedproduct necessitating a separate screening operation for removal.

FIGURE 2 represents a single stage dryer consisting of a jet mixer 11,an elongated drying pipe 12 and a cyclone separator 13. The jet mixerdischarges into the 'elongaged pipe which, in a preferred embodiment, is

equipped with jacket 14 having inlet 15 and outlet 16. The diameter ofthe pipe preferably is the same as that of the discharge end of thediffusion zone. This diameter should be small enough to maintain thehigh velocity of the hot gas through the length of the drying pipe. Thevelocity of the gas must be sufiicient to maintain the feed material insuspension and the Reynolds number of the drying pipe should exceed5000. The high velocity of the gas also causes considerable contactbetween the tacky particles as they form and the walls of the pipe. Thisforces the particles to roll over each other to some extent and formlarger, more rounded particles. In addition, the high velocity preventsthe particles from actually adhering to the walls of the apparatus. Thejacketed pipe can be heated by steam or other external heat source toprevent condensation of the solvent vapors on the walls of the pipe andto conserve the amount of hot vapors used in the jet mixer by providingheat through the pipe wall to offset that used to vaporize the solvent.Additionally, the pipe may contain more than one jacket so as to'allowfor careful regulation of the temperature in the drying pipe along itslength. For example, the first stage of a dryer section may be heated toremove volatiles while a second stage may be cooled so that the productmay be packaged directly.

At the discharge end of the pipe, a device such as a paratus of thisinvention suitable for drying a wet particulate materlal. The apparatuscomprises a gas entry duct 18, a jet nozzle 19, a mixing chamber 20, anda diffusion zone 21. The hot gas enters the mixing chamber through duct18 and its velocity is substantially increased as it passes through jetnozzle 19. The wet particulate material is passed into the mixingchamber at an angle converging with the stream of hot gasses. An angleof 90 is preferred. Any method known to those skilled in the art may beused to pass the particulate material into the mixing chamber. Forexample, a vibrator, not shown, may be fastened to hopper 22.Alternatively, gravity feed may be used. The wet particles enter themixing chamber and are entrained in the hot gas stream. The suspensionof particles and hot gas move into diffusion zone 20 wherein a morehomogeneous mixture is obtained. The mixture is then passed into adrying zone such as the one illustrated in FIGURE 2. The dry particlesare collected in the same manner as the particles obtained fromsolution.

For many operations, it is desirable to employ a multistaged dryingapparatus such as the one embodied in FIGURE 4 wherein a two-stageapparatus is shown. In this embodiment, feed material, either in theform of solution or wet particles enters a first-stage jet mixer throughduct 101. The hot gas stream enters through duct 102 and a suspension ofthe feed material in the hot gas if formed. This suspension is passedinto a first-stage drying pipe 103. This drying pipe may be equippedwith a jacket 105 to regulate temperature. The suspension travels athigh velocity through the drying zone wherein the solvent is vaporized.The prodnot from this stage is collected in a cyclone separator 107. Thehot gas and vaporized solvent leave the cyclone separator throughexhaust line 108. The particulate material is further dried by passagethrough a second jet mixer 109 modified for particulate material asshown 7 in FIGURE 3. The wet particulate material is suspended in astream of hot gas which enters the second jet mixer through line 110.This suspension is passed into a second-stage dryer 111 which may beequipped with jacket 112. The dry particulate material along with thevaporized solvent and hot gas passes into a cyclone separator 113. Thedry powder is collected at the bottom of the cyclone separator employinga suitable discharge valve 114 if necessary. The hot gas and vaporizedsolvent leave the cyclone separator through line 108. If the solvent iswater, it may be vented to the atmosphere. However, if the solvent is avaluable organic solvent, it may be recovered in condenser 115 or anyother manner known to those skilled in the art and collected in acondensed solvents tank 120.

There are many advantages to the use of a multistaged drying system suchas that of FIGURE 4. One major advantage is that drying efliciency ismuch higher as the hot gas stream can be renewed periodically to avoidsaturation with solvent. A third or additional stage may be used as wellas a final stage wherein a coolant is added to the jacket rather thansteam so that the final product may be cooled. When the final stage isused to cool the product, a cool carrier gas should be used to conveythe powder. A system of this nature is applicable for materials thatrequire a short exposure to high temperature for drying but must bepackaged at a low temperature. Additionally, the drying pipe may be usedto convey the product from the manufacturing portion of the plant to itspacking section.

The process and apparatus of this invention can be employed to dry awide variety of materials provided they are able to withstand elevatedtemperatures for short periods of time. The temperature of the hot gasstream should be as high as possible as this promotes greater dryingefficiency. However, the temperature used is dependent upon the materialto be dried. For example, a material dissolved in a low boiling solventmay be dried at a temperature lower than that which could be used for amaterial dissolved in a high boiling solvent. In general, thetemperature of the hot gas stream entering the first jet mixer shouldexceed the boiling point of the liquid to be volatized by at least 25 F.and preferably, by at least 50 F.

The gas used as the hot gas stream is dependent upon the material to bedried. If a material such as a resin dissolved in a water insolublesolvent is to be dried, superheated steam at a pressure of between 50p.s.i.g. and 200 p.s.i.g. is the preferred hot gas stream as itcondenses readily and forms a second layer with the solvent which iseasily'removed. If the material to be dried is one that is watersoluble, air or nitrogen may be used as the drying gas provided it isnon-reactive with the material to be dried.

When the feed material is in solution form, the particle size of theproduct can be regulated'by either adjustment of the ratio of hot gas tofeed material or control of the concentration of solids in the feedsolution or a combination of the two. It has been found that decreasingthe ratio of hot gas to feed or the increasing concentration of solidsin solution favors the formation of large particles.

The ratio of hot gas to feed material may vary over broad limitsdependent upon the material to be dried, the solids content of the feed,the temperatures of the hot gas stream, etc. The lower limit for thisratio should be such that the flow of hot gas in suflicient to (1)volatize enough solvent to form a non-tacky particle and (2) rapidlydrive the feed material through the entire length of the drying pipe. Ingeneral, there is no upper limit for this ratio, though at high ratios20to 1, the particles at quite small and hard to collect.

The solids content of the feed material can also vary over very widelimits depending upon the material to be dried, the drying conditions,etc. In general, for most resin solutions, solids contents of 1 to 20percent, by weight, are preferred.

That particle size can be controlled using the process and apparatus ofthis invention is quite unexpected as jet spray drying, which bears theclosest resemblance to the present invention, does not provide forcontrol of the particle size. In the normal jet spray drying operation,the product collected is normally in the form of a fine, dust-likepowder having particle sizes in the submicron range.

The following are examples of the drying method and apparatus of thisinvention. All percentages expressed in the examples are by weight. Theexamples are set forth merely for purposes of illustration and are notto be considered limiting in any way.

EXAMPLE 1 This example is designed to illustrate how particle size of apolycarbonate resin may be controlled through adjustment of the ratio ofthe hot gas stream to the feed stream.

The apparatus employed was similar to that depicted in FIGURE 2. The jetmixer had about A" diameter nozzle which discharged into a mixingchamber and diffusion zone which had a diameter of A" at the entry endand A" diameter at its exit end. The drying stage consisted of a 16 footlength of 1" inside diameter steel schedule 40 pipe. A three foot lengthof this pipe had a steam jacket. The hot gas employed was steam whichentered the jet nozzle through a 1" inside diameter schedule 40 steelpipe. The drying zone discharged into a cyclone separator and the resinwas collected from the cyclone separator in a steel drum.

Steam was used as the hot gas. A 15% solution of a polycarbonate derivedfrom 2,2-bis-(4-hydroxyphenyl)- propane dissolved in methylene chloridewas used as the feed. Three runs were made. In each run, the steam flowrate was maintained at approximately 837 lb./hr. The feed flow rate wasvaried as follows:

Run No Lb./hr. 1 64.9 2 232 The three runs corresponded to steam to feedweight ratios of 12.9/ 1, 3.6/1 and 1.2/1 respectively. The product wascollected for each of the runs and a sieve analysis performed. Thefollowing results were obtained:

TABLE I.SIEVE ANALYSIS FOR DRY POLYCARBONATE POWDER Rate of hot gas tofeed U.S. sieve No. Percentage retained on screen 8 It can be readilyseen that as the ratio of the steam to feed decreases, the particlesbecome larger in size.

EXAMPLE 2 Using the apparatus of Example 1, three additional runs weremade to demonstrate how particle size of the product can be regulatedthrough adjustment of the con centration of dissolved solids in the feedmaterial. As in the previous example, steam maintained at approximately837 lb./hr. was used as hot gas. The feed. consisted of a solution of apolycarbonate derived from 2,2'-bis-(4-hydroxyphenyl)-propane dissolvedin methylene chloride. The feed flow rate was maintained atapproximately 697 lb./hr. for run 1 and 644 lb./hr. for runs 2 and 3.This corresponded to steam to feed ratios of l.2/1.0 for run 1 and1.3/1.0 for runs 2 and 3. The percent dissolved polycarbonate in thefeed was as follows:

Run No Percent 1 15.0 2 11.8 3 4.9

The product was collected for each run and a sieve analysis performed.The following results wereobtained:

TABLE II.SIEVE ANALYSIS FOR DRY POLYCARBONATE POWDER Percent dissolvedsolids in feed U.S. sieve No. Percentage retained on screen From theabove, it is apparent that particle size decreases as the percentage ofdissolved solids in the feed solution decreases.

EXAMPLE 3 Using the apparatus and procedure of Example 1, six more runswere performed. The polycarbonate employed was the same as in Example 1,but the solvent consisted of 95.8 parts methylene chloride and 4.2 partsheptane. The solution contained 9.3 percent dissolved polycarbonate. Foreach run, steam flow was maintained at 837 lbs./ hr. The feed flow ratewas varied as follows:

Run No. Lb./hr. 1 418 2 779 The six runs corresponded to steam to feedWeight ratios of 2.0/1.0, 1.1/1.0, 0.9/1.0, 0.8/1.0, 0.7/1.0, and0.5/1.0. The product was collected for each run and a sieve analysisperformed. The following results were obtained:

TABLE III.-SIEVE ANALYSIS FO R D RY POLYCARBONATE POWDER Rate of hot gasto feed 2.0/1.0 1.1/1.0 (1.9/1.0 0.8/1.0 0.7/1.0 0.5/1.0 U S. sieve 0.Percentage retained on screen The results of this example againillustrate how particle size can be controlled by regulation of theratio of hot gas to feed.

9 EXAMPLE 4 Run No. Lb./h-r.

The three runs corresponded to steam to feed ratios of 6.6/1.0, 3.4/1.0,and 2.8/1.0 respectively. The product was collected for each run and asieve analysis performed. The following results were obtained:

TABLE IV.SIEVE ANALYSIS FOR DRY POLYCARBONATE POWDER Rate of hot gas tofeed Percentage retained on screen U.S. sieve No.

H n-uk NM F PP DKQWGHNQDO EXAMPLE 5 Using the apparatus and procedure ofExample 1, two additional runs were performed with a feed solutioncomprising 9.1 percent polystyrene in methylene chloride. The steam flowwas maintained at 837 lb./hr. for both runs and feed flow was maintainedat 239 lb./hr. for the first run and 558 lb./hr. for the second run.These rates corresponded to steam to feed ratios of 3.5/1.0 and 1.5/1.0,respectively. Sieve analysis was performed for each of the runs and thefollowing results obtained:

TABLE V.-SIEVE ANALYSIS FOR DRY POLYSTYRENE POWDER Rate of hot gas tofeed U.S. sieve No. Percentage retained on screen EXAMPLE 6 This exampleis designed to show the use of a system consisting of three stagessimilar to that depicted in FIGURE 4. The first stage consisted of a jetmixer that delivered 1150 lb. of steam per hour from nominal 1S0p.s.i.g. utility saturated steam service. The diffusion throat of thejet mixer had an inside diameter of A and diverged to discharge smoothlyinto a 2" schedule 40 drying pipe 80 in length. This drying pipe washeated with a steam jacket using 150 p.s.i.g. saturated steam. Thedrying pipe vented into a cyclone separator which vented the vapor anddischarged the particles into a hopper for sampling and subsequentdrying in a second stage.

The second stage consisted of a jet mixer for handling solid particlessimilar to that shown in FIGURE 3. The particles [flowed by gravitythrough a funnel shaped inlet which tapered to an inside diameter of 3"at the inlet to the mixing chamber. The hot gas was 600 lbs. of steamper hour supplied from the 150 p.s.i.g. utility saturated steam service.The dilfuser expanded from a throat diameter of 1 /2 to dischargesmoothly into a nominal 3 diameter schedule 40 drying pipe which was360' long and heated along its length with steam jackets. The dryingpipe discharged into a second cyclone separator which vented the vaporand fed the particles to a third stage similar to the second stage. Theproduct from the third stage was collected in a hopper for sampling. Allcyclone separators were vented through a condensing system whereversolvent was recovered.

The feed to the first stage consisted of a polycarbonate resin dissolvedin a solvent consisting of methylene chloride and 25% heptane (byvolume) to form a solution containing 9% solute. A feed rate of 6.2gallons per minute (4000 lbs. of solution per hour) was maintained foran eight-hour run. This provided a. steam to feed ratio of 6.29/ 1.0.Four random samples were taken after the first stage and four after thethird stage with the following results:

First-stage product, Third-stage product, percentage volatile percentagevolatile 30.0 1.3

What is claimed is:

1. An apparatus capable of drying a feed material to the form ofdiscreet, dense particles of predetermined particle size, said apparatushaving at least one drying stage, said drying stage having incombination a jet mixer and an elongated drying pipe in communicationtherewith, said jet mixer having a hot gas entry duct, a jet nozzlehaving a Reynolds number of at least 10,000 and capable of substantiallyincreasing the velocity of a hot gas stream passing through said gasentry duct, a mixing chamber at the discharge end of said jet nozzlehaving means for introduction of a feed material to the discharge end ofthe jet nozzle at an angle converging with said discharge end, and adiffusion zone at the discharge end of said mixing chamber, saiddiffusion zone diverging at an angle that allows limited expansion of agas stream passing therethrough while maintaining contact between thegas stream and the walls of the diffusion zone, said gas entry duct, jetnozzle, mixing chamber and diffusion zone being in axial alignment.

2. A process for drying a material in the form of dense, discreetparticles which comprises continuously passing a stream of a hot gasthrough a jet nozzle at acoustic velocity, angularly projecting a feedmaterial into the stream of hot gas as it discharges from the jet nozzleto form a suspension of feed material in the hot gas, passing saidsuspension into a confined diffusion zone wherein the suspension isallowed limited expansion while maintaining contact with the walls ofthe diffusion zone, advancing said suspension through an elongated pipeand separating volatiles from the formed particulate material.

3. A process for drying a material in the form of dense, discreetparticles which comprises continuously passing a stream of a hot gas,through a jet nozzle to substantially increase its velocity, angularlyprojecting a feed material into the stream of hot gas as it dischargesfrom the jet nozzle to form a suspension of feed material in the hotgas, passing said suspension into a confined diffusion zone wherein thesuspension is allowed limited expansion while maintaining contact withthe walls of the diffusion zone, advancing said suspension through anelongated pipe, separating volatiles from the formed particulatematerial, passing the particulate material into a second drying 1 1 12stage wherein the particulate material is angularly pro- 2,639,132 5/1953' Bradford. jected into a hot gas stream travelling at acousticalveloc- 2,746,735 5/ 1956 Bradford. ities, passing the particulatematerial through a difiusion 3,056,212 10/1962 Jamison 34-10 zone andthereafter continuously passing the particulate 3,309,785 3/1967 King.material through a drying pipe.

5 EDWARD G. FAVORS, Primary Examiner References Cited US. Cl. X.R.

UNITED STATES PATENTS 34 57 Re. 17,212 2/1929 Stockton.

2,297,726 10/1942 Stephanofi. 10

